Lithium secondary batteries have conventionally been used for various applications such as a power source of an electronic device and a battery mounted on a vehicle. The lithium secondary battery may experience a minute internal short circuit due to repeated charge and discharge. This may result in a reduced capacity of the secondary battery to shorten the battery life.
It is contemplated that one of the causes of the internal short circuit is volume expansion of lithium in a negative electrode because of atomization of lithium due to repeated charge and discharge. Specifically, the volume of lithium deposited at the negative electrode may be expanded to press and thin a separator, or lithium seeking space for deposition may be deposited into fine pores in the separator to reach a positive electrode. This may cause electrical contact between the positive electrode and the negative electrode to lead to the internal short circuit.
To suppress the volume expansion of lithium due to the deposition, the techniques described below have been proposed.
Patent Document 1 describes a lithium secondary battery in which a spacer is placed between an electrode (a positive electrode or a negative electrode) and a separator to reserve a space for accommodating deposited lithium. In the lithium secondary battery described in Patent Document 1, the separator is formed of a stack of a plurality of nonwoven fabrics having different void rates such that the void rate near the negative electrode is higher than that near the positive electrode.
Each of Patent Documents 2 and 3 describes a lithium secondary battery in which a collector for a negative electrode has a bumpy surface opposed to a separator to provide a space for accommodating deposited lithium.
[Patent Document 1] Japanese Patent Publication No. 10 (1998)-12279 (Paragraph [0015] to [0017], [0032], [0034], FIGS. 13, 14 and the like)
[Patent Document 2] Japanese Patent Publication No. 2000-228185 (Paragraph [0011] and the like)
[Patent Document 3] Japanese Patent Publication No. 2002-313319 (Paragraph [0009], [0010] and the like)
[Patent Document 4] Japanese Patent Publication No. 06 (1994)-84512